JPS6261766A - Continuous casting method with less central segregation - Google Patents

Abstract

PURPOSE:To improve quality by successively rolling down an ingot to decrease the section of the ingot by the amt. corresponding to the volume shrinkage based on the shrinkage on solidification from the prescribed position in the casting direction of a residual molten metal pool up to the foremost end, then executing casting. CONSTITUTION:Anvil groups 5 are disposed above and below a roller table 4 from the position of 2-15mm before the foremost end in the casting direction of the molten metal pool 2 up to the foremost end of the pool 2. The anvils 5 are connected to a hydraulic compressing system and is moved back and forth like walking beams; at the same time, the anvils exert the compression of the prescribed amt. per unit length to the solidified shell 1. The ingot is thereby successively rolled down by as much as the amt. corresponding to the volume shrinkage based on the solidification of the liquid core of the ingot. The central segregation of a slab or bloom is thus decreased and the quality is improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for continuous casting of steel with less center segregation, and more specifically, to reduce center segregation and improve low-temperature toughness in steel sheets caused by center segregation. The present invention relates to a method for improving the resistance to melatyers, HIC resistance, etc., and further improving the wire breakage rate and coppery rupture rate of hard steel wire rods.

[Conventional technology]

In order to reduce the center segregation of continuously cast slabs and blooms, it is necessary to prevent the occurrence of bulging by setting an appropriate roll interval, arranging the rolls, or performing appropriate secondary cooling.

On the other hand, reduction of the superheating degree of molten steel, addition of coolant to the mold, application of electromagnetic stirring to the hot steel in the mold, application of ultrasonic waves to the slab in the strand, and furthermore, the application of electric 8i to the molten steel in the strand. Techniques for reducing center segregation by methods such as the application of stirring are widely used. All of these methods aim to make the cast structure equiaxed, to achieve fine dispersion of solutes, and to reduce center segregation.

Although reducing the degree of superheating of molten steel is effective in achieving equiaxed weight, it requires controlling the casting temperature within a narrow range, which has the disadvantage of impeding operational stability.

Addition of a coolant is also effective in achieving equiaxed loading, but the melting of the coolant may leave a residue or cause mold slag to be rolled up, which has the drawback of easily causing UT defects.

The application of ultrasonic waves to slabs is effective in principle, but as an implementation technique, there is a problem of fatigue of the application roll, which makes it difficult to put into practical use. Electromagnetic stirring within the strand has few practical drawbacks and is effective in producing equiaxed products, so it has become widely used. However, although it is recognized that center segregation tends to be reduced due to equiaxial gravity propagation due to electromagnetic stirring, in reality, the mechanical properties of thick steel plate products made from continuously cast slabs to which electromagnetic stirring is applied are Compared to thick steel plate products made of the same material that are not subjected to electromagnetic stirring, there is no particularly noticeable improvement, and the fracture rate of hard steel wire rods made from continuously cast blooms to which electromagnetic stirring is applied is also lower than that when electromagnetic stirring is applied. No significant improvement effect was observed compared to wire rods obtained from materials that were not coated.

[Problem that the invention seeks to solve]

The present invention relates to continuously cast slabs or blooms,
The present invention provides a method for eliminating center segregation and producing sound slabs.

FIG. 4 shows the longitudinal cross-sectional macrostructure of a bloom slab for hard steel wire.Since the slab was cast at a low temperature, an equiaxed load is formed on the lower surface side over the entire area from the lower surface to the axis. On the other hand, on the upper surface side, the range of 150 mm from the upper surface is branched columnar crystals, and the interior thereof is equiaxed. Hollow-shaped cavities are formed intermittently in the shaft center along the casting direction. A characteristic feature of bloom slabs is that they are accompanied by ■ segregation near the axial center, which is different from the V segregation that occurs at the axial center of the steel ingot, and is rather similar to the form of reverse V segregation in the steel ingot. have (2) Segregation occurs in an area approximately 100 mm wide around the axis. Figure 2 shows the statistical distribution of semi-macro analysis values near the axis of slabs of the same size. From this, the central segregation and the adjacent negative segregation zone are clearly recognized. From this figure, the region where negative segregation begins to occur is within 40 mm from the axis. That is, it can be seen that there is bulk solute movement in a region with a width of 80 mm centered on the axis. The area where this solute movement occurs is under the actual casting conditions under which the slab was cast.
In terms of the position of the slab along the casting direction, it corresponds to a position from 8 to 10 m from the most advanced point (crater end) of the molten steel pool within the slab to the most advanced point of the molten steel pool. Furthermore, this range naturally changes if the slab cross-sectional dimensions, PI production speed, cooling conditions, etc. change, but under actual continuous casting conditions, the range is 2 to 15 minutes from the leading edge of the molten steel pool.
It corresponds to the range from m to the cutting edge of the molten steel pool. It is clear from the surface analysis of V segregation that such movement of the solute-enriched molten steel occurs along the V segregation line as shown in FIG. Furthermore, it is clear from metallurgical observations and simple numerical calculations that this movement of solute-enriched molten steel is caused by the suction force caused by the solidification contraction of the remaining molten steel in the molten steel pool. be able to.

Therefore, in order to prevent center segregation, the movement of solute-enriched molten steel generated by the suction force caused by the solidification shrinkage of the remaining molten steel in the molten steel pool is necessary to prevent center segregation. It is to prevent it.

[Means for solving problems]

The technical means of the present invention for solving the above problems is to move the liquid core of the slab along the casting direction from a position 2 to 15 m before the leading edge of the residual molten metal pool in the casting direction to the pool's leading edge position. As the solidification of the core progresses, the core is successively rolled down by an amount equivalent to the amount of volumetric shrinkage due to solidification shrinkage, and the slab is cast with its cross section reduced.

[Effect]

The present invention compensates for the shrinkage caused by the solidification of the remaining molten steel in the molten steel pool, that is, the solidification of the liquid core of the slab, by compressively deforming it from the outside of the slab. movement and prevent center segregation. The shaded area in Figure 3 shows the amount of shrinkage due to solidification shrinkage of the remaining molten steel in the molten steel pool, that is, compressive deformation strain, calculated for various casting conditions such as slab cross-sectional area, pouring speed, and cooling water ratio. It's within.

Next, when trying to compensate for the shrinkage of the liquid core core of the slab by applying compressive strain from the outside to the solidified slab shell, the strain that effectively acts on the liquid core core is determined by the method of applying compressive strain from the outside. It also depends on the applied strain, but it decreases to 1/2 to 1/10 of the applied strain. This can be expressed as the total efficiency η.0 For example, when trying to apply external strain to a billet slab with an aspect ratio of about 1.4 by compressing it in the thickness direction, it has been experimentally verified that the total efficiency becomes η = 0.2. It was revealed by

Therefore, in order to prevent center segregation, it is necessary to successively reduce the volumetric shrinkage due to solidification of the liquid core core of the slab. In other words, it is necessary to sequentially apply an external strain obtained by dividing the compressive deformation strain shown in FIG. 3 by the total efficiency from a position a predetermined distance before the leading edge of the molten steel pool toward the leading edge of the molten steel pool.

For example, in the case of bloom slabs, the molten steel pool protrudes greatly beyond the pinch rolls, the bottom surface is supported by table rollers, and both sides and the top surface are free spaces without any support, so the first As shown in the figure,
A group of five anvils connected to the hydraulic pressure and compression system and capable of repeated back and forth movements similar to a walking beam are installed, and as mentioned above, the amount of compression per predetermined unit length is applied along the casting direction. That's fine.

〔Example〕

A bloom slab for hard wire with a slab cross section of 560 x 400 mm was continuously cast at a casting speed of 0.55 m/min and a water ratio of 0.551/kg. 2 from meniscus of Example (A strand)
Five groups of anvils shown in FIG. 1 were installed in a range of 3 to 29 m, and the compressive strain shown in Table 1 was applied to cast. As a comparative example, strands B, C, and D were cast using a conventional method.

Note that the circles shown in FIG. 3 indicate the compressive strain equivalent to the amount of volumetric shrinkage required under the casting conditions of this embodiment. In addition, under this M construction condition, the leading edge of the molten steel pool is 28 mm from the meniscus.
．． It has been separately confirmed that the location is 5 m, and bloom slabs were taken as samples from each strand.
In addition to investigating center segregation, PCj is
Table 2 shows the results of manufacturing ll wires and comparing their cementite ratings and cuppy fracture surface ratios. This clearly shows that the present invention is extremely superior.

〔Effect of the invention〕

The present invention reduces center segregation of continuous casting slabs and blooms, improves low temperature toughness, lamellar tear resistance, and HIC resistance.
It is possible to improve the strength of the wire rod, as well as the cutting m ratio and the cubic rupture rate of hard steel wire rods.

[Brief explanation of drawings]

Figure 1 is a layout diagram of a group of compressive deformation anvils to reduce central segregation, Figure 2 is a graph showing semi-macro segregation near the axis of the slab, and Figure 3 is a graph showing semi-macro segregation in the vicinity of the molten steel pool. A graph showing the compressive deformation strain necessary to compensate for the amount of shrinkage,
FIG. 4 is a photograph of the metallographic structure showing the macrostructure of the hard steel wire plume. l... solidified shell? ... Molten steel pool, 3... Pinch roll, 4... Roller table, 5... Anvil.

Claims (1)

[Claims] 1. In continuous casting of molten metal, along the casting direction from a position 2 to 15 m before the leading edge of the residual molten metal pool in the casting direction to the pool's leading edge position, as the liquid core core of the slab solidifies, A continuous casting method with less center segregation characterized by successively reducing the volumetric shrinkage due to solidification shrinkage and casting by reducing the cross section of the slab.